1. Field of the Invention
The present invention relates to a printer apparatus having a print head which mixes and discharges a discharge medium and a feed medium from a nozzle. In more detail, it relates to a printer apparatus which prevents the discharge medium and feed medium from remaining at the nozzle opening face of the print head and enables the formation of an image having a highly precise expression of tones and a high resolution.
2. Description of the Related Art
In recent years, particularly in offices, preparation of documents using a computer, referred to as "desk-top publishing", is now a frequent practice. Recently, also, there have been increasing requests for the output of not only characters and simple graphics, but also color natural images like photographs together with the characters and graphics. Along with this, there have been demands for printing high quality natural image. Therefore, reproduction of halftones has become important.
Further, so-called "on-demand" type printer apparatuses print an image on a printing medium such as a paper or film with ink droplets discharged from the nozzle only when necessary at the time of printing in accordance with a printing signal. This type of printer apparatuses has been rapidly spreading in recent years since it is possible to reduce their size and cost.
A variety of methods have beer proposed as the method of discharging the ink droplets in this way, but the ones generally used are the method of using a piezoelectric element or the method of using a heat generating element. The former is the method of applying pressure to the ink by the deformation of the piezoelectric element to discharge the ink. The latter is the method of using a heat generating element to heat and cause the ink to boil and cause the ink to discharge by the pressure of the bubbles generated.
A variety of methods have also been proposed as the method of reproducing halftones by the above on-demand type printer apparatuses discharging ink droplets. Namely, as a first method, there can be mentioned a method in which the tone is expressed by controlling the size of the droplets to be discharged by changing the voltage and pulse width of a voltage pulse to be given to the piezoelectric element or the heat generating element, so as to make the diameter of the print dots variable. Note that, at this time, the on-demand type printer apparatus discharges one droplet per one pulse of the voltage.
According to this method, however, when the voltage and pulse width to be given to the piezoelectric element or the heat generating element are made too low, the ink can no longer be discharged, and therefore there is a limit to how small the diameter of the droplets can be made, the number of stages of tones which can be expressed is small, and particularly the expression of a low density is difficult. Thus, this method is insufficient for printing out a natural image.
Further, as a second method, there can be mentioned a method in which one pixel is comprised by a matrix consisting of, for example, 4.times.4 dots without a change of the dot diameter, and tones are expressed by using the so-called dither method in these matrix units. Note that in this case it is possible to express 17 tones.
Upon printing by this method with, for example, the same dot density as that of the first method, however, the resolution is one fourth of that of the first method, so the roughness is remarkable. Therefore, this method is insufficient to print out a natural image.
In view of this, the present inventors proposed in U.S. patent application Ser. No. 08/622005 now pending a so-called "carrier jet" system printer apparatus wherein the ink is mixed with a diluent when discharging it, so as to change the concentration of the ink droplets to be discharged and enable the control of the density of the dots to be printed and thereby express tones without occurrence of a deterioration of resolution and print out a natural image.
The print head of an on-demand type printer apparatus to which a so-called "carrier jet" system of mixing the ink and diluent and discharging the mixture, as described above, is applied has a first pressure chamber into which a diluent as the discharged liquid is introduced and a second pressure chamber into which an ink as the feed liquid is introduced, and has at least one set of nozzles, each comprised of a first nozzle formed so as to be communicated with the first pressure chamber and a second nozzle formed so as to be communicated with the second pressure chamber adjacent to each other, in a mutually adjoining manner.
Upon printing by the print head, first pressure is applied to the ink in the second pressure chamber by the piezoelectric element or the heat generating element to squeeze out a predetermined amount of ink from the second nozzle toward the first nozzle. Next, the pressure in the second pressure chamber is reduced and the ink is retracted into the second nozzle so that only the predetermined amount of ink remains on the first nozzle side.
Subsequently, pressure is applied to the diluent in the first pressure chamber by the piezoelectric element or the heat generating element, to discharge a mixture of the diluent and the predetermined amount of ink from the first nozzle so as to carry out the printing.
In such a print head, however, after the predetermined amount of ink is squeezed out of the second nozzle and then the ink is retracted, ink sometimes remains between the first nozzle and the second nozzle at the nozzle opening face of the print head.
When the ink remains at the nozzle opening face in this way, it exerts an influence upon the ratio of mixture of the ink and diluent at the next printing, which results in a reduction of the precision of concentration of the droplets of the mixture of ink and diluent, a reduction of the precision of the density and tones of the image formed, and also a reduction of the resolution.
In the above printer apparatus, the density of the dots to be printed is determined according to the ratio of mixture of the ink and the diluent. The tones are expressed by this. Therefore if the ink remains on the nozzle opening face as described above, this remaining ink will exert an influence upon the ratio of mixture of next printing, so that accurate densities of the dots at the next printing and tones of the image to be formed cannot be obtained, and consequently the resolution is deteriorated.
Further, if the ink remains on the nozzle opening face, there exists a possibility that this remaining ink will exert an influence upon the direction of discharge of the mixture and will lower the precision of the image forming position.
Furthermore, if the ink remains on the nozzle opening face, there is a possibility that the diluent in the first nozzle and the ink in the second nozzle will end up being connected by this remaining ink. If the discharge of the feed ink or diluent is carried out so as to perform the next printing in this state, naturally there will be an influence on the ratio of the mixture, result in inaccuracy in the concentration of droplets of the mixture of ink and diluent, and density and tones of the image to be formed, so that a high resolution of the image cannot be obtained.
Further, where the print head has a plurality of sets of nozzles as mentioned above, there also exists a possibility that the ink extruded from the second nozzle of a certain set of nozzles will intrudes into the diluent in the first nozzle in the adjoining set of nozzles and that there will be interference between liquids discharged from adjacent nozzles so that accuracy in density and tones of the formed images, and the resolution are deteriorated.